Nematocarcinus ovalis, Komai & Segonzac, 2005

Komai, Tomoyuki & Segonzac, Michel, 2005, Two new species of Nematocarcinus A. Milne-Edwards, 1881 (Crustacea, Decapoda, Caridea, Nematocarcinidae) from hydrothermal vents on the North and South East Pacific Rise, Zoosystema 27 (2), pp. 343-364 : 355-362

Nematocarcinus ovalis n. sp.

( Figs 8-10 )

HOLOTYPE. — HOPE 99 , EPR 13°N, RMT (remotely monitored trawl) 1, stn 54803#3, 20.IV.1999, 12°45.50’N, 103°55.80’W, 2558-2619 m (c. 120 m above bottom), 1 ovigerous cl 27.5 mm (MNHN- Na 15765).

ETYMOLOGY. — The Latin ovalis (oval) refers to the distinctly oval shaped posteroventral spots on the sixth abdominal somite.

DISTRIBUTION. — Known with certainty only from the type locality at EPR 13°N, 2558-2619 m.

DESCRIPTION OF HOLOTYPE

Body moderately slender; integument soft, fragile, not membranous, surface smooth, shiny.

Rostrum ( Fig. 8A, B ) moderately slender, distal part broken off, preserved part nearly reaching anterior margin of antennal scale; preserved dorsal margin armed with 23 small teeth, including nine on carapace posterior to orbital margin, posterior 13 teeth with complete basal suture, others with incomplete basal suture or fixed; distance between teeth becoming wider distally; lateral surface with blunt lateral ridge confluent with orbital margin; ventral margin of preserved part unarmed. Carapace ( Fig. 8A, B ) with post-rostral ridge low, sharp, extending to about 0.40 of carapace length, ending slightly anterior to cervical

A groove; lateral surface rather well sculptured with distinct postorbital, cervical, post-cervical, hepatic, branchiocardiac grooves; cervical groove across midline of carapace, thus shallow, conspicuous notch present on dorsal surface; branchiocardiac groove deep, not accompanied with sharply delineated ridge; orbital margin sinuous, with broadly rounded suborbital lobe; antennal spine short, falling short of level of anterior margin of basicerite; pterygostomial spine broader than antennal spine; anterolateral margin between antennal, pterygostomial spines oblique, slightly sinuous.

Thoracic sternum with three bilobed prominences on sixth to eighth somites; lobes of prominence on sixth somite each terminating rounded; lobes of posterior two prominences also rounded, not prominently elevated.

Abdomen ( Fig. 8C ) dorsally smooth. Pleura of anterior four somites broadly rounded. Posterodorsal margin of third somite somewhat produced posteriorly, rounded ( Figs 8C ; 9A ). Fifth somite ( Fig. 8C ) with one small posteroventral teeth, not markedly produced; inner surface of posterior part of pleuron with low, distinct short ridge ( Figs 8D ; 9B ). Sixth somite ( Fig. 8C, D ) moderately elongated, 2.22 times longer than fifth somite, 2.50 times longer than anterior depth; posterolateral process subtriangular, acutely pointed; posteroventral angle blunt; ventral surface with two single, subparallel rows of long setae extending to level of posterior margin of posteroventral spots; posteroventral spots ( Figs 8D , 9C ) small, longitudinally elongate suboval in outline, surfaces minutely punctate; pre-anal tooth ( Fig. 8D ) directed posteriorly. Telson ( Fig. 8E ) long, slender, 5.00 times longer than greatest width, reaching posterior margin of exopods of uropods (except for terminal spines), gradually tapering posteriorly to blunt terminal lobe; dorsal surface flat, with six pairs of small dorsolateral spines; all dorsolateral spines aligned; terminal margin with two pairs of spines, lateral pair strongest (tips of spines broken off).

Eye ( Fig. 8B ) subpyriform with well-developed, faceted cornea; cornea moderately large for nematocarcinid, its width about 0.10 of carapace length; eye-stalk slightly shorter than corneal width.

Antennular peduncle ( Fig. 8B ) stout, reaching midlength of antennal scale. First segment subequal in length to distal two segments combined; stylocerite only reaching midlength of basal peduncular segment, acuminate, directed forward in lateral view. Flagella broken off.

Antennal peduncle ( Figs 8B , 9D ) with basicerite stout, armed with small ventrolateral distal tooth; ventral surface of basicerite provided with low, rounded tubercle. Fifth segment (carpocerite) short, stout, reaching level of midlength of first segment of antennular peduncle. Antennal scale 0.83 of carapace length, 4.10 times longer than wide; lateral margin slightly sinuous; distolateral tooth reaching truncate distal margin of lamella ( Fig. 9E ). Flagellum missing.

Mouthparts not dissected, but generally similar to those of N. burukovskyi n. sp. Third maxilliped ( Fig. 10A ) with endopod not reaching distal margin of antennal scale, consisting of four segments; ultimate segment somewhat compressed laterally, lance shaped, terminal spines missing; mesial face of ultimate segment with numerous transverse or obliquely transverse tracts of stiff setae of various length, forming grooming apparatus; carpus (penultimate segment) 1.33 of ultimate segment length, with some tufts of short to moderately long setae on surfaces; antepenultimate segment (merusischium-basis fused segment) flattened dorsoventrally, sinuous in dorsal view, with row of 10 spinules on longitudinal ridge on lateral surface, margins or surfaces setose; coxa with rudimentary epipod (not figured; broken during dissection); exopod about 0.80 length of antepenultimate segment.

Pereopods with articulations between merus and ischium strongly oblique, typical for nematocarcinids.

First pereopod ( Fig. 10B ) moderately slender, overreaching antennal scale by length of chela and 0.10 of carpus; dactylus ( Fig. 9F, G ) 0.47 of palm length, terminating in acute, curved claw, cutting edge with two rows of blunt corneous spinules and short curved setae in distal 0.30; fixed finger also terminating in curved claw, crossing dactylus, with prominent tuft of long setae subproximally, cutting edge with two rows of short curved setae. Palm with tuft of long setae at articulation to dactylus and small patch of grooming setae on ventromesial surface. Carpus widened distally, 3.85 of chela length. Merusischium combined 1.06 of carpus length, 0.75 of carapace length; merus slightly widened distally, with one small ventrolateral spine arising from proximal 0.30; ischium subequal in length to merus, armed with three spines ventrolaterally (subproximal two spines close, third spine at midlength).

Second pereopod ( Fig. 10C, D ) long, filiform, shorter than posterior three pereopods. Chela moderately small. Dactylus ( Fig. 8H ) 0.42 of palm length, terminating in acute, curved claw obscured by numerous short setae, cutting edge with two rows of short curved setae; fixed finger also terminating in curved claw obscured by short setae, crossing dactylus, with two prominent tufts of setae at midlength of lateral and ventral surfaces, cutting edge with two rows of short stiff setae. Palm with tuft of long setae at articulation to dactylus; carpus strongly elongated, slightly widened distally, 7.90 of chela length. Merusischium combined 0.95 of carpus length, 1.56 of carapace length; merus slightly widened distally, with six small, widely spaced ventrolateral spines; ischium shorter than merus, armed with one subdistal spine ventrolaterally. Exopod moderately short.

Third to fifth pereopods greatly elongated, filiform, similar in length and structure. Third pereopod ( Fig. 10E, F ) overreaching antennal scale by about 0.40 of merus length. Dactylus ( Fig. 9I, J ) about 0.75 of propodus length, slightly sinuous in ventral view, tapering distally, with short ridge on lateral margin proximally and with faint annulations in distal 0.40; basal part somewhat widened. Propodus ( Fig. 9I ) very short, subcylindrical, slightly widened distally, with row of individual or tufts of setae on dorsal and ventral surfaces; distal margin obliquely truncate, with row of long stiff setae dorsally; armament of ventrodistal margin missing. Carpus very thin, greatly elongated, about 13 of propodus length. Merus-ischium combined 1.90 of carapace length; articulation between merus and ischium notably inflated; merus with five ventrolateral spines, six mesial spines, spines widely separated; ischium with one ventrolateral spine subdistally. Exopod moderately long. Fourth pereopod (overreaching antennal scale by about 0.25 of merus length; merus with two ventrolateral and four mesial spines; exopod short. Fifth pereopod overreaching antennal scale by about 0.25 of merus length; dactylus missing; merus unarmed; exopod absent.

Number of gills and epipods identical to that of N. burukovskyi n. sp. ( Table 1). Exopods on pereopods notably decreasing in length posteriorly; length of each exopod moderate for genus. Epipods similar to those of N. burukovskyi n. sp.

Uropod ( Fig. 8C ) with protopod terminating posterolaterally in small acute tooth; exopod slightly longer than endopod, with distinct suture; lateral margin of exopod straight, terminating in small movable spine.

Coloration

Generally reddish.

Size

Ovigerous female: cl 27.5 mm; tl c. 108 mm.

REMARKS

Nematocarcinus ovalis n. sp. closely resembles N. faxoni in the dorsal armament of the rostrum, the shape of the posterodorsal margin of the third abdominal somite and the relatively small posteroventral tooth of the fifth abdominal pleuron. Nematocarcinus ovalis n. sp. can be distinguished from N. faxoni by the relatively small, more elongately oval-shaped posteroventral spots on the sixth abdominal somite (cf. Fig. 8D and Fig. 11B ) and by the distinct inner ridge on the fifth abdominal pleuron ( Fig. 8B ). The inner ridge of the fifth abdominal pleuron is very low, sometimes rudimentary in N. faxoni ( Fig. 11D ). There is only a single setal row on either side of the midline of the ventral surface of the sixth abdominal somite in N. ovalis n. sp. ( Fig. 8D ), whereas it is partially doubled in N. faxoni ( Fig. 11B ). The body integument of N. ovalis n. sp. is soft, but that of N. faxoni is rather solid. The preserved part of the rostrum of N. ovalis n. sp. is unarmed on the ventral margin ( Fig. 8B ), whereas the rostrum of N. faxoni is usually armed with one or two teeth between the levels of the distal margin of the antennular peduncle and the distal margin of the antennal scale ( Fig. 11A ). Nematocarcinus ovalis n. sp. and N. faxoni are found at different depths; the holotype of N. ovalis n. sp. was collected at a depth of 2558- 2619 m, N. faxoni at 898-1839 m, most abundantly at 1100-1300 m ( Burukovsky 2001, 2003). Other than N. faxoni , two species of Nematocarcinus have been reported from the Eastern Pacific, i.e. N. agassizii Faxon, 1893 , and N. ensifer Smith, 1882 ( Faxon 1895; Burukovsky 2001). Nematocarcinus ovalis n. sp. is distinguished from N. agassizii by characters of the rostrum and the sixth abdominal somite ( Faxon 1895; Burukovsky 2003). The dorsal margin of the rostrum is armed with moderately spaced teeth over the entire length in N. ovalis n. sp., whereas it is armed with closely set teeth restrict- ed proximally to the level of the distal margin of the antennular peduncle in N. agassizii . The longer axis of each posteroventral spot on the sixth abdominal somite is parallel to the midline in N. ovalis n. sp., rather than diverging posteriorly in N. agassizii . The strongly produced, subacutely pointed posterodorsal margin of the third abdominal somite and the strong, ventrally curved posteroventral tooth on the fifth abdominal pleuron distinguish N. ensifer from N. ovalis n. sp.

An additional specimen (MNHN-Na 15763) from hydrothermal vents on the EPR 13°N, collected during the HOT 96 cruise ( Gaill et al. 1996) (DS Nautile, dive 1070, 21.II.1996, at about 300 m of the hydrothermal site Genesis, EPR, 12°48.64’N, 103°56.43’W, 2630 m, slurp gun) was examined. It is an ovigerous female (cl 21.4 mm), but its condition is poor. The rostrum was missing during collection. The specimen is similar to the holotype of N. ovalis n. sp., but is different from the latter in some features. The posterodorsal margin of the third abdominal somite is more strongly produced in the HOT 96 specimen than in the holotype of N. ovalis n. sp (cf. Fig. 12A, B and Figs 8C ; 9A ). The posteroventral spots on the sixth abdominal somite are less elongated in the HOT 96 specimen than in the holotype of N. ovalis n. sp (cf. Fig. 12E and Fig. 9C ). Each setal row only extends to the anterior margin of the posteroventral spot in the HOT 96 specimen ( Fig. 12E ), rather than extending to the level of the posterior margin of it in the holotype of N. ovalis n. sp. ( Fig. 9C ). It has been shown that the complex structure of the setal rows and posteroventral spots is constant among the species of Nematocarcinus ( Burukovsky 2003) , and therefore the observed differences may be species specific. The fifth abdominal pleuron of the HOT 96 specimen is armed with a sharp, more slender posteroventral tooth and an additional, minute denticle on the posterolateral margin dorsal to the posteroventral tooth in both right and left ( Fig. 12C ). In the holotype of N. ovalis n. sp., the posteroventral tooth is less produced and there is no additional denticle on the posterolateral margin ( Fig. 8C ). Furthermore, the HOT 96 specimen is smaller than the holotype of N. ovalis n. sp. (cl 21.4 mm versus cl 27.5 mm), although they are both ovigerous. A suite of the above characters also distinguishes the HOT 96 specimen from known congeneric species, as well as the other four Eastern Pacific species, N. agassizii , N. ensifer , N. faxoni and N. burukovskyi n. sp. The poor condition of the HOT 96 specimen prevents making further comparisons, but the observed differences would seem to suggest that the HOT 96 specimen represent an undescribed species. A formal description of a new species should be deferred until additional specimens are collected.

ECOLOGY

The holotype of N. ovalis n. sp. was collected by a trawl sampling in the water column about 120 m above the bottom, and about 300 m distant from the hydrothermal vent site Genesis ( Gaill et al. 1996). The trawl catches of the trawl operation contained typically bathypelagic animals, and lack any obvious components derived from vent fauna.

Individuals of Nematocarcinus shrimp were frequently observed at and around the Genesis site ( Gaill et al. 1996). The indeterminate HOT 96 specimen was collected at a location 300 m from the Genesis site by slurp gun on pillow lavas, where the fauna is rather poor. Only holothurians and sponges Caulophacus cyanae Boury- Esnault & de Vos, 1988 were sparsely found at the location.

Figures

FIG. 8. — Nematocarcinus ovalis n. sp., holotype ovigerous, HOPE 99, stn 54803#3, NEPR,cl 27.5 mm (MNHN-Na 15765); A, rostrum, carapace and cephalic appendages, lateral view (distal part of rostrum, left eye and distal part of left antennal scale broken off; carapace somewhat deformed, thus dorsal notch more accentuated than real); B, anterior part of carapace, rostrum and cephalic appendages,lateral view; C, third abdominal somite to telson and left uropod, lateral view; D, posterior part of fifth abdominal somite and sixth somite, ventral view; E, telson, dorsal view. Scale bars: A-C, 5 mm; D, E, 2 mm.

FIG. 9. — Nematocarcinus ovalis n. sp., holotype ovigerous, HOPE 99, stn 54803#3, NEPR, cl 27.5 mm (MNHN-Na 15765); A, third abdominal somite, dorsal view; B, posterior part of left fifth abdominal pleuron, ventral view, showing inner ridge; C, posteroventral spots and posterior setal pits of sixth abdominal somite, ventral view (setae omitted); D, right antenna, ventral view (setae omitted); E, distal part of left antennal scale, ventral view; F, chela of right first pereopod, lateral view; G, distal part of dactylus of right first pereopod, lateral view; H, chela of left second pereopod, lateral view; I, dactylus and propodus of right third pereopod, lateral view; J, dactylus of right third pereopod, obliquely ventral view. Scale bars: A, D, 5 mm; B, C, F, H, I, J, 1 mm; E, 2 mm; G, 0.5 mm.

FIG. 10. — Nematocarcinus ovalis n. sp., holotype ovigerous, HOPE 99, stn 54803#3, NEPR, cl 27.5 mm (MNHN-Na 15765); A, right third maxilliped, lateral view; B, right first pereopod, lateral view; C, merus and ischium of left second pereopod, lateral view; D, chela and carpus of left second pereopod, lateral view; E, merus and ischium of right third pereopod, lateral view; F, dactylus, propodus and carpus of right third pereopod, lateral view. Scale bar: 5 mm.

FIG. 11. — Nematocarcinus faxoni Burukovsky, 2001, paratype, off Marias Island, Mexico, cl 25.6 mm (USNM 21139); A, rostrum, anterior part of carapace and cephalic appendages, lateral view; B, posterior part of fifth abdominal somite and sixth somite, ventral view (setae omitted); C, posteroventral spots and posterior setal pits on sixth abdominal somite, ventral view; D, posterior part of right fifth abdominal pleuron, ventral view. Scale bars: A, 5 mm; B, 2 mm; C, D, 1 mm.

FIG. 12. — Nematocarcinus sp., ovigerous, site Genesis, HOT 96, DS Nautile dive 1070, cl 21.4 mm (MNHN-Na 15763); A, third to fifth abdominal somites, lateral view (setae omitted); B, third abdominal somite, dorsal view; C, left fifth abdominal pleuron, lateral view; D, sixth abdominal somite, ventral view (setae omitted); E, posteroventral spots and posterior setal pits, ventral view. Scale bars: A, B, 5 mm; C, D, 2 mm; E, 1 mm.